The present invention relates to a circuit breaker equipped with an overcurrent trip device.
FIG. 14 is a circuit diagram of a prior art circuit breaker as disclosed in Published Unexamined Application Japanese Patent No. 32211/1985.
In this figure, terminals 101, 102, 103 on the side of a three-phase power supply are connected with the power supply. These terminals 101, 102, 103 are connected with terminals 301, 302, 303 on the load side via contacts 201, 202, 203, respectively, which connect or disconnect loads with the circuit.
Electrical paths 11, 12, 13 are formed between the power supply terminals 101, 102, 103 and the load terminal 301, 302, 303, respectively. Current transformers 21, 22, 23 for detecting electric currents at different phases are inserted in the electrical paths 11, 12, 13, respectively. Full-wave rectifier circuits 31, 32, 33, connected with the secondary sides of the transformers 21, 22, 23, respectively, are used to derive the absolute values of the outputs from the secondary sides.
Shunt circuits 41, 42, 43 are connected with the outputs of the full-wave rectifier circuits 31, 32, 33, respectively. Each shunt circuit is fabricated by connecting a resistor, for example, across the DC output terminals of one of the full-wave rectifier circuits 31, 32, 33. The output terminals of the shunt circuits 41, 42, 43 are connected with respective signal converter circuits 91, 92, 93 which derive the effective values or average values of the output signals induced in the shunt circuits 41, 42, 43, respectively.
To an OR circuit 160, formed of diodes 161, 162, 163, are applied the output signals from the signal converter circuits 91, 92, 93, respectively. The output terminal of the OR circuit 160 is connected with an analog-to-digital converter circuit 100 which converts the output signal from the OR circuit into digital form. The greatest one of the output signals from the converter circuits 91, 92, 93 is fed to the A/D converter circuit 100 via the OR circuit 160. The output from the converter circuit 100 is applied to a microcomputer 110.
A thyristor 120 is triggered with the output signal from the microcomputer 110. When this thyristor 120 is turned on, an overcurrent trip device 80 of the release type is driven to mechanically open the contacts 201, 202, 203 which were closed.
An OR circuit 130 is connected to the output terminals of the full-wave rectifier circuits 31, 32, 33 which are at positive potential. The OR circuit 130 is composed of diodes 131, 132, 133. The output terminals of the full-wave rectifier circuits 31, 32, 33 which are at negative potential are connected with a common potential point or ground point.
The output terminal of the OR circuit 130 is connected to a power circuit 500 for both the A/D converter circuit 100 and the microcomputer 110. The power circuit forms a power source for operating these components. The OR circuit 130 produces a signal corresponding to the maximum value of the currents flowing through the AC paths 11, 12, 13. The output of the 0R circuit 130 is connected via a zener diode 140 with a timer circuit 150 whose output terminal is connected with the gate of the thyristor 120.
When the load-connecting contacts 201, 202, 203 are closed, electric power is supplied from the power terminals 101, 102, 103 to the load terminals 301, 302, 303, respectively, via the contacts 201, 202, 203. Under this condition, if overload currents flow through the AC paths 11, 12, 13, then the transformers 21, 22, 23 for the different phases detect the overload currents at their intrinsic ratios of current transformation and induce output currents on the secondary sides.
These output signals are transformed into direct currents by the full-wave rectifier circuits 31, 32, 33, respectively, and supplied to the shunt circuits 41, 42, 43, respectively. The waveforms of the signal voltages induced in the shunt circuits 41, 42, 43 at this time are well known waveforms of absolute values. The output signals from the shunt circuits 41, 42, 43 are converted into signals corresponding to their effective or average values by the signal converter circuits 91, 92, 93, respectively, at their respective phases.
The maximum value of the effective or average values obtained by the signal converter circuits 91, 92, 93 is fed to the A/D converter circuit 100 via the OR circuit 160. The converter circuit 100 converts the analog signal applied to it in this way into digital form. The resulting digital signal is supplied to the microcomputer 110, which determines the level of the digital signal in accordance with a given program.
Then, the microcomputer produces a timeout signal after a fixed period of time according to the result of the decision made as described above, and delivers an output signal from its output port 116. This output signal appearing at the port 116 of the microcomputer 110 is applied to the gate of the thyristor 120, thus triggering it into conduction. This actuates the overcurrent trip device 80 of the release type to open the load-connecting contacts 201, 202, 203 via actuating devices (not shown) and release device (not shown). The contacts 201-203 mechanically interlock with the trip device 80. As a result, the AC paths 11, 12, 13 are broken.
Meanwhile, the voltage signals corresponding to the accidental currents and induced in the shunt circuits 41, 42, 43 are applied to the OR circuit 130 composed of the diodes 131, 132, 133. Since the output of the OR circuit 130 is connected with the timer circuit 150 via the zener diode 140, if the level of the output from the OR circuit 130 exceeds the zener voltage of the diode 140, then a signal is applied to the timer circuit 150.
The timer circuit 150 generates a timeout signal after a fixed period of time in response to the input signal to thereby trigger the gate of the thyristor 120. This activates the overcurrent trip device 80 of the release type. As a result, the circuit breaker rapidly breaks the electrical paths. In this conventional configuration, the power circuit 500 is connected in parallel with the shunt circuits 41, 42, 43. The maximum voltage at each phase is fed to the power circuit 500. The electric power from the power circuit 500 is supplied to both the microcomputer 110 and the A/C converter circuit 100.
In the prior art circuit breaker constructed as described thus far, portions of the currents appearing at the secondary sides of the current transformers 21-23 flow into the power circuit 500, the transformers 21-23 acting to detect currents. Therefore, the currents flowing through the shunt circuits 41, 42, 43 fail to correspond to the currents flowing through the AC paths 11, 12, 13 at different phases. Hence, an error arises in detecting the level of overload current. Further, the current flowing into the power circuit 500 is not constant. Consequently, it is difficult to correct the error produced in detecting the level of overload current.
Also, the output power from the power circuit 500 is not sufficient to drive the microcomputer 110 and the A/D converter circuit 100 the moment the load-connecting contacts 201, 202, 203 are closed. Therefore, these control circuits may malfunction.